1. Field of Invention
The present invention relates primarily to optics, optics systems and generators, and more particularly to optical conductors characterized by the composition of the material. This invention also relates to coherent light generators and amorphous (e.g. glass) optical fibers. More specifically, the present invention includes an improvement to the ZBLAN glass system involving the incorporation of thorium fluoride as a stabilizer against devitrification permitting increased concentrations of rare earth dopants, in making optical fibers.
2. Description of the Related Art
Heavy metal halide glasses offer a variety of significant advantages over silicate glass systems. Theoretically, fluoride glasses can have intrinsic absorption losses that are two to three times lower than those of silica. These lower losses make fluoride glasses ideal candidates for telecommunication uses. In addition, these glasses are transparent out to wavelengths of six or seven microns, allowing them to be used in the mid-infra red region of the spectrum for sensing applications and power delivery. The low energy phonon spectra also yield relatively high quantum efficiencies for many rare earth transitions that are quenched in silica glasses. When formulated with a sufficient amount of rare earth ions, these glasses could be attractive hosts for active fibers. A survey of the development of heavy metal fluoride glasses, their properties, and their uses is set forth by Martin G. Drexhage in Chapter 4 of the Treatise of Materials Science and Technology, Vol. 26 (1985), pp. 151-243.
Among the more potentially useful of the heavy metal halide glasses is the ZBLAN, which consists essentially of the fluorides of zirconium, barium, lanthanum, aluminum, and sodium. U.S. Pat. No. 4,674,835, Fluoride Glass Optical Fiber ("MIMURA") (assigned to Kohusai Denshin Denwa Co., Ltd.), filed Aug. 14, 1985, offers a representative formulation of ZBLAN (in mole percent): EQU 50.ltoreq.ZrF.sub.4 .ltoreq.55 EQU 16.ltoreq.BaF.sub.2 .ltoreq.24 EQU 16.ltoreq.NaF.ltoreq.24 EQU 3.ltoreq.LaF.sub.3 .ltoreq.5 EQU 2.ltoreq.AlF.sub.3 .ltoreq.4
provided that ZrF.sub.4 +BaF.sub.2 +NaF+LaF.sub.3 +AlF.sub.3 =100. (Col. 3, lines 27-30.)
Unfortunately, heavy metal fluoride glasses also exhibit certain undesirable properties which, until now, have limited their applications. Heavy metal fluoride glasses have poor resistance to crystallization devitrification, and MIMURA mentions the crystallization problems of ZBLAN and the scattering problems that result therefrom.
The poor resistance to devitrification also creates problem in making large preforms. Crystallization at the interface between the core and cladding during the manufacture of a preform causes problems in the most commonly used methods for making a ZBLAN optical fiber. (See, for example, Danh C. Tran, George H. Sigel, Jr., and Bernard Bendow. "Heavy Metal Fluoride Glasses and Fibers: A Review," Journal of Lightwave Technology, Vol. LT-2, No. 5, October 1984, pp. 566-586. See also, Y. Ohishi, et al., "Fabrication of Fluoride Glass Single-Mode Fibers," Journal of Lightwave Technology, Vol. LT2, No. 5, October 1984, and "Advanced Rod-in-Tube Techniques for Fluoride Fiber Fabrication," presented at the 93rd Annual Meeting of the American Ceramic Society, a symposium on optical materials, held in Cincinnati, Ohio (April 28-May 2, 1991); Tran, et al., "Fluoride Glass Preforms Prepared by a Rotational Casting Process," Electronics Letters, 23nd Vol. 18, No. 15, July 1982; and S. Mitachi, et al., "Preparation of Fluoride Optical Fibers for Transmission in the Mid-infrared," Physics and Chemistry of Glasses, Vol. 23, No. 6, December 1982.) That is, heavy metal halide glasses are prone to inhomogeneous nucleation, consequentially resulting in crystallization at the core and cladding interfaces, particularly during the drawing of a optical fiber. Resulting fibers tend to have serious scattering loss due to the crystals in the fiber.
The problem of crystallization is aggravated when the necessary ions to give the core and cladding different indexes of refraction are added to the glasses. Additional doping, for example with rare earth ions, also tends to reduce the stability of the glass.
Prior to the invention described herein, a satisfactory solution to the problem of poor resistance to devitrification in ZBLAN glass systems has not been discovered.